1. Field of the Invention
This invention relates to a color transformation look-up table, which represents a correspondence relationship between image signals for a non-self-luminous displaying medium, such as a photograph, and image signals for a self-luminous displaying medium, such as a cathode ray tube (CRT) display device or a liquid crystal display device, and is to be referred to when the image signals for the non-self-luminous displaying medium and the image signals for the self-luminous displaying medium are to be transformed to each other such that the appearances of perceived colors may become identical between images displayed on the non-self-luminous displaying medium and the self-luminous displaying medium. This invention also relates to a method and apparatus for forming the color transformation look-up table. This invention further relates to a method and apparatus for transforming colors of an image by use of the color transformation look-up table.
2. Description of the Prior Art
Hard copy images, which have been recorded on non-self-luminous displaying media, such as sheets of film or photographs, have heretofore been read out with scanners, or the like, and digital image signals having thus been obtained have heretofore been furnished to users. Also, hard copy images, such as images on prints, have heretofore been reproduced from digital image signals, which have been acquired with digital cameras, or the like, by users, and the thus reproduced hard copy images have heretofore been furnished to the users. The digital image signals, which have been furnished to the users, or the digital image signals, which have been acquired with the digital cameras, or the like, can be utilized by the users for reproducing and displaying soft copy images on self-luminous displaying media, such as CRT display devices or liquid crystal display devices connected to, for example, personal computers. Further, the users can reproduce the soft copy images from the furnished digital image signals, can display the reproduced soft copy images on CRT display devices, or the like, and can carry out processing, such as addition of character patterns or symbol patterns, on the displayed images. From the digital image signals having thus been processed by the users, hard copy images can be reproduced.
Soft copy images, which are displayed on the self-luminous displaying media, such as CRT display devices, are reproduced with image reproduction systems, which are quite different from the image reproduction systems for hard copy images. It has been known that, in cases where a soft copy image and a hard copy image are reproduced from the same image signals and displayed respectively on a self-luminous displaying medium and a non-self-luminous displaying medium, the colors of the two images are visually perceived as different colors. In order for such problems to be eliminated, there have heretofore been proposed various techniques for transforming the colors of an image such that the appearances of perceived colors may become identical between a hard copy image and a soft copy image.
For example, as for how an image is to be transformed by considering the viewing conditions (such as the correlated color temperature of the white color, the luminance, and the ambient conditions) such that the same color appearances may be obtained when the image is viewed under certain conditions and when the image is viewed under different conditions, it has been proposed to employ transformation techniques which are ordinarily referred to as the chromatic adaptation transformation or chromatic adaptation models (i.e., color appearance models). The transformation techniques are described in, for example, Japanese Patent Publication Nos. 7(1995)-86814 and 7(1995)-86815; "Color Research and Application," Volume 19, Number 1, 1994, R. W. G. Hunt; "Color Research and Application," Vol. 20, No. 3, 1995, N. Nayatani; and "Color Research and Application," Vol. 16, No. 4, 1991, M. D. Fairchild. As an appearance model, a Von Kries's chromatic adaptation prediction formula (a Von Kries's rule) is known as a basic one among various chromatic adaptation prediction formulas ("Color Engineering" by Noboru Ota, publishing office of Tokyo Denki University).
In Japanese Unexamined Patent Publication No. 10(1998)-173943, the applicant proposed a method for color compensation, which is carried out when colors of an image are transformed in accordance with the chromatic adaptation model described above such that the appearances of perceived colors may become identical between images, the method being capable of compensating for colorimetric errors and inconformity of color appearances. With the proposed method, at least one patch of a color is outputted as a hard copy image, and at least one patch of a color is outputted as a soft copy image, such that the patches may have conforming calorimetric values. Also, the patches of the two images are visually compared with each other from a viewing position of the person, who views the patches. In cases where the colors of the patches of the two images are perceived as being different, a patch having calorimetric values slightly shifted from the conforming colorimetric values is outputted with respect to at least one of the two images, and the visual comparison is again made. Such operation is repeated until the colors of the patches of the two images are perceived to be identical. Color compensation is then made with respect to at least one of the two images in accordance with the calorimetric values of the patches of the two images, which are perceived to be of the same color. In this manner, the inconformity of the color appearances is compensated for.
Ordinarily, in cases where the transformation of image signals in accordance with the chromatic adaptation model is carried out, the image signals, which the display device processes directly, are temporarily transformed into image signals of a calorimetric system (a second colorimetric system), which does not depend upon the display device, and the image signals having been obtained from the temporary transformation are subjected to the transformation of the image signals in accordance with the chromatic adaptation model. The image signals, which have been obtained from the transformation in accordance with the chromatic adaptation model, are then returned to the image signals of the original colorimetric system (a first colorimetric system). As the techniques for transforming image signals into image signals of a different calorimetric system, various techniques for accurately carrying out the signal transformation have heretofore been proposed. For example, a technique for calculating an inverse transformation relationship from an analytic formula with repeated operations by using a Newton-Raphson technique (an N-R technique) has been proposed in "Photographic Science and Engineering," Vol. 16, Num. 2, Mar.-Apr. 1972, pp. 136-143. Also, as a method capable of accurately transforming color signals from the second calorimetric system to the first calorimetric system by utilizing a direct transformation relationship for the transformation of image signals, which a display device processes directly, into image signals of a colorimetric system, which does not depend upon the display device, the applicant proposed a method of carrying out an inverse transformation operation with the N-R technique in Japanese Unexamined Patent Publication No. 9(1997)-9086.
With the method proposed in Japanese Patent Application No. 7 (1995)-156555, a plurality of color patches differing in color stepwise are subjected to colorimetric determination, and a relationship between color signals (CMY or RGB) and stimulus value signals XYZ is thereby found. Also, the method of least squares is applied to the thus found relationship, and a relationship between first virtual color signals and first virtual stimulus value signals is thereby found. The relationship having been found in this manner is set as a direct transformation table. Thereafter, a first inverse transformation table is formed from the direct transformation table by utilizing the Newton technique. Further, a second inverse transformation table is formed from the first inverse transformation table by replacing the relationship between the first virtual color signals and the first virtual stimulus value signals in the first inverse transformation table by a physically matching relationship between second virtual color signals and second virtual stimulus value signals. An inverse transformation table is then obtained by mapping the second inverse transformation table, and stimulus value signals XYZ are transformed into color signals (CMY or RGB) by utilizing the obtained inverse transformation table.
It has also been known that the color reproduction range varies for a soft copy image and a hard copy image. In cases where signal transformation between image signals for a self-luminous displaying medium and image signals for a non-self-luminous displaying medium is carried out in accordance with the chromatic adaptation model described above, problems often occur in that certain colors cannot be reproduced and displayed on either one displaying medium. As one of techniques for solving such problems, processing (Gamut mapping, hereinbelow referred to as the Gamut processing) for mapping the image signals, which fall outside the color reproduction range of the one displaying medium, to signals falling within the color reproduction range. Also, in Japanese Unexamined Patent Publication No. 10(1998)-117294, the applicant proposed a Gamut outside signal estimating method as a method of transforming colors, in which Gamut processing is carried out such that, even at regions in the vicinity of the boundary of the color reproduction range, an unnatural feeling or a large variation may not occur in colors of an image reproduced and displayed in accordance with image signals after being transformed.
With the method proposed in Japanese Unexamined Patent Publication No. 10(1998)-117294, after an inverse transformation relationship, which is the relationship of color signals CMYK with respect to stimulus value signals XYZ within the color reproduction range, has been found, the color signal K constituting the color signals CMYK at the boundary of the color reproduction range is set to be a color signal falling outside the color reproduction range. Also, color signals CMY falling outside the color reproduction range are estimated with the multiple regression analysis technique by using the color signals CMY at the boundary of the color reproduction range, and a different inverse transformation table is thereby formed. By use of the thus formed inverse transformation table, stimulus value signals XYZ are transformed into color signals CMYK. Thereafter, the color signals CMYK are mapped to signals falling within the color reproduction range by using the Gamut mapping technique described above.
As described above, the techniques for transforming colors of an image such that the appearances of perceived colors may become identical between a hard copy image and a soft copy image and the techniques for solving the problems due to a difference in color reproduction range between the hard copy image and the soft copy image have heretofore been proposed independently. However, there has not yet been proposed an idea for applying the aforesaid techniques as a simple method or a simple apparatus to a system for reproducing a soft copy image from digital image signals, which have been acquired from a hard copy image, or reproducing a hard copy image from digital image signals, which are used for reproducing a soft copy image.